This invention relates to a trigger circuit having upper and lower threshold voltage levels.
Presently, trigger circuits are used in a wide variety of fields, and are expected to fulfill various requirements according to several objects. In U.S. Pat. No. 3,514,633, there is disclosed a trigger circuit having two cross-coupled transistor pairs and capable of independently adjusting two threshold levels with high accuracy. In this trigger circuit, each transistor pair is formed of an input transistor and a current-switching transistor of different conductivity types. One input transistor is so connected as to receive a trigger signal at its base, while the other input transistor has its base connected to reference potential. The emitters of these input transistors are connected to a common constant current source via their respective emitter resistors. The current-switching transistors of the two transistor pairs have their respective bases connected to the collector of the input transistor of each corresponding pair and their collectors cross-coupled to the emitter of the input transistor of each other pair.
In this circuit, the first and second paired transistors are on and off respectively in one of two stable states, and are off and on in the other. Accordingly, input impedance makes a remarkable change when the circuit is switched from one stable state to the other. Moreover, the threshold levels of the aforementioned circuit depend on the values of the emitter resistors and current supplied from the constant current source. Accordingly, if the current supplied from the constant current source changes with supply voltage, the threshold levels will also change therewith.